This invention relates to an FM receiver comprising a circuit arrangement for determining the field strength of FM transmitters, including an FM demodulator, particularly to an FM receiver evaluating the Radio Data System signals.
A problem in such receivers is that the quality, i.e. particularly the field strength of other transmitters is to be checked during reception of signals from a first transmitter. The reception of the first transmitter should of course be interrupted for a short time such that this interruption is not audible in the ideal case. This particularly occurs in receivers evaluating the RDS signals transmitted by most FM transmitters. These RDS signals carry information with regard to eventual alternative frequencies, i.e. other FM transmitters than the one upon which the receiver is actually tuned (hereinafter referred to as actual transmitter) which transmit the same program as the actual transmitter, but at different frequencies, and upon which tuning may be changed over to when the reception quality, i.e. particularly the field strength of the actual transmitter, gets too bad.
In addition to the time necessary for evaluating, the FM receiver requires a given period of time for the short-time change-over or frequency jump to an alternative FM transmitter frequency, the length of this period of time being dependent on the value of the frequency difference between the actual transmitter and an alternative transmitter whose field strength is to be checked. However, since this frequency difference is not always known or utilized, it will be necessary to start in any case from the maximum frequency jump so that the waiting time elapsing between the frequency setting of the transmitter to be checked and the possible evaluation of its field strength should always be the same as if a large frequency jump had taken place. This means that this waiting time is always relatively long.